The invention concerns a solar collector for the generation of high temperatures, with a housing tub, a glass plate sealing the solar collector and a selective absorber, with a vacuum prevailing in the cavity of the solar collector.
Concerned in the case of the solar collector for the generation of high temperatures is a solar collector which is operated at large temperature differences between the absorber, for one, and the area outside the solar collector for another.
It is previously known to use solar collectors for gaining energy, specifically thermal energy for the heating of liquids. In doing so, the incident sunlight falls on a black absorber which to approximately 95% converts the incident light energy to heat. A fluid flowing through the black absorber transports the thermal energy to a place where it is stored or used.
On the side away from the incident sunlight, the black absorber is thermally well insulated. On the side of the solar collector facing toward the sun, the insulation against thermal losses is more difficult to realize. In most flat collectors, losses occur through convection of the air contained in the solar collector and through infrared radiation of the hot absorber. Known from the company catalog of thermo-solar Energietechnik GmbH 6/87 is a solar collector which on its side facing toward the sun features a glass plate. The incident sunlight is unimpeded and the thermal reflection emanating from the absorber is extensively prevented by the glass plate from exiting. For a simple glass plate, 25% retransmission must be expected for the thermal reflection, which is lost in terms of heat generation.
While double glazing reduces the thermal loss by retransmission further, the transmission for the incident sunlight is worsened. With each interjected additional glass plate, the efficiency of the solar collector is reduced more than what is gained by reduction of thermal losses.
Therefore, the heat transfer between the absorber and the exterior of the solar collector, through the glass plate, has already been reduced in other ways. Described in Solar Collector by W. B. Gillette and J. E. Moon, D. Reidel Publishing Company (1985), for the solar energy R & D in the European community (Series A, Volume 6) is the effect of a selective black absorber. A selective black absorber is absorbent to the incident sunlight and highly reflective for the thermal radiation produced in the absorber. The heat radiation to the outer glass is thereby reduced by 90%.
But especially in the high temperature range above 100.degree. C. temperature difference between absorber and the area outside the solar collector, the efficiencies accomplished with these improvements are still very low.
A remedy is offered by high-vacuum solar collectors such as known from Applied Solar Energy by A. B. and M. P. Meinel (Addison-Wesley, 1977). Here, the selective absorber is melted into a highly evacuated glass tube. The evacuated space between the absorber and the cover glass avoids any convection of air and heat transmission by the air, thus increasing the efficiency. At the addressed temperature differences of more than 100.degree. C., efficiencies of 50% to 60% are achievable. The use of a high vacuum is disadvantageous since the integrity of the vacuum must be maintained over the entire operating time. This makes high-vacuum solar collectors very expensive to operate and difficult to handle.
Another possibility of solution is known again from the company catalog of thermo-solar Energietechnik GmbH 6/87. Here, the cavity between the outer glass plate and the selective absorber is evacuated lightly so as to reduce the convection of the air in the solar collector. High efficiencies have already been achieved thereby, but the efficiency continues to drop greatly especially at great temperature differences between the area outside the solar collector and the absorber.
The article "Minimum Thermal Conductivity of Transparent Insulation Materials" by A. Pfluger in Solar Energy Materials 16, p. 255-265 (1987) examined theoretically the thermal conductivity of thermal insulation materials that are transparent to light radiation.
Described in the German patent disclosures 28 35 371 and 28 35 372 each is a solar heater where honeycomb type thermal insulating materials or such formed by tubelets are attached to the covering glass plate. The transparent thermal insulation on the inside of the glass plate suppresses the air convection and thermal radiation. Known from the German patent disclosure 28 23 449 is another solar heater for the heating of gases where the above honeycomb structure is translucent or totally reflective.
The German patent disclosure 27 40 448 describes the use of foam glass as insulation and construction material for solar collectors.
The German patent disclosure 31 50 251 teaches a solar collector with an evacuated interior whose cover plate is supported against thermal and compressive loads with the aid of support plates that are impermeable to radiation.
The thermal losses in such systems are often reduced by an additional air gap between the selective absorber and the honeycomb structure. This reduces the risk of damage through high temperatures. However, there are limits to the width of the air gap and, thus, the reduction of thermal losses: depending on temperature, convection of air sets in at a gap width of a few centimeters.
Basing on this prior art, the problem underlying the invention is to provide a solar collector of the initially mentioned type that has a high efficiency at large temperature differences between the absorber, for one, and the area outside the solar collector, for another.